I have a similar problem... built a new pc had an ssd inside. Installed Win 10 and it worked flawlessly for 2 days. Then my hdd arrived connected it to my motherboard and the power supply started the computer and then it said that the ssd wasn't bootable... what to do?
After adding an HDD to my new computer, it does not recognize the ssd as a bootable device on which my OS is.
Win 10, Intel Core I5 6400/2700 Skylake, Samsung SSD 120 GB EVO 850, SeaGate Baracuda Compute 2 TB HDD